Treated effluents become one of the most important sources for irrigation and other activities in arid and semi arid countries such as Oman. This study focuses more on characterizing the quality of domestic wastewater in selected three regions: Muscat, Sohar and Salalah. The wastewater samples were collected from six different sewage treatment plants (STPs): Rusayl STP, Ansab STP, Darsayt STP, Sohar STP, Salalah STP, and Salalah lagoon STP. The sampling program continued over a period of one year approximately starting in February 2009. The samples were collected from different sampling points in each STP. The tested parameters are pH, electrical conductivity (EC), Biochemical oxygen demand (BODs), chemical oxygen demand (COD), ammonia, cations(Aluminum (Al), Iron (Fe), Manganese (Mn), Copper (Cu), Molybdenum (Mo), Lead (Pb), Nickel (Ni), Silicon Si, Zink (Zn), Vanadium (V), Barium (Ba) and Cobalt (Co)), anions (Fluoride, Chloride, Nitrite, Bromide, Nitrate, Phosphate and Sulfate), sodium and potassium and solids gravimetric tests (total solids (TS), total suspended solids (TSS) and total dissolved solids (TDS)). The results revealed that, the raw sewages in all STPs were categorized as high strength concentration. The values of pH for RSs were in the range of 6 to 7.5 for the entire sampling points in the whole six selected STPs. Furthermore, the range of pH values for TEs was 6.5 to 7.5. The TES EC values for entire six STPs were within the OS range of 2000 to 2700 us/cm. Both BODs and COD values of TE were within the recommended limits by OS of 15 to 20 mg/L and 150 to 200 mg/L, respectively. The average raw sewage ratio of BODs/COD in overall STPs was 25%. Ammonia concentrations had exceeded the acceptable range in most of selected plants except Sohar and Salalah STPs. Fluoride, Chloride, Phosphate and Sulfate concentrations were within the acceptable ranges of OS. Nitrate concentration in TEs of Ansab and Sohar STPs exceeded the recommended value by OS. The average obtained values of the entire set of tested heavy metals are within the stated limits by OS for TEs except for Mo. In Ansab and Salalah Lagoon STPs, the average concentrations of Mo were found 0.06 to 0.07 mg/L which exceeded the acceptable range of 0.01 to 0.05 mg/L as stated by OS. The Na values of TEs samples were within the recommended range by OS of 200 to 300 mg/L. The range of K values was from 5 to 50 for RSs and from 5 to 25 mg/L for TEs. The range of TSS in RSS was from 100 mg/L to 1200 mg/L. The TSS in TEs was above the acceptable range of 15 to 30 mg/L. The values of TDS for RSs were in the range of 500 to 2000 mg/L approximately and the values of TDS for TEs were within the suitable range of 1500 to 2000 mg/L according to OS. The correlation between TDS and EC was established for both RSs and TEs samples. On the other hand, there was good relationship between measured and calculated TS for both R$s and TEs samples. The obtained values for E Coli were all acceptable except for Rusayl STP. Generally, the produced treated effluents satisfied OS; however there were minor exceptions for certain parameters. The performances of Salalah and Darsayt STPs can be classified as the best compared to other STPs.